Geochemistry Of Volcanic Rocks In The Jinta Formation Of North To Alxa Block And Its Tectonic Implications

The Alxa Block is located at the intersection region of the the Tarim Plate,the North China Plate,and the Qilian Orogenic Belt,which is an important part of Central Asian Orogenic Belt?CAOB?and has a very important geotectonic location and plays an irreplaceable role in the study of the CAOB.It is also the key area to study the final closure of the Ancient Asian Ocean,which is of great significance to limit its closure time.Previous studies on the north to Alxa Block have made a lot of achievements on structural unit division,ophiolite and granite,etc.However,the tectonic-sedimentary evolution of the north to Alxa Block and the closure time of CAOB and the study of other important scientific questions is weak and controversial.Based on field geological survey and previous research results,this paper studies the basalt-rhyolite biomodal volcanic rocks in the Jinta Formation in the northern part of the Engger Us ophiolite belt in the northern part of Alxa Block by using mineralogy,petrology,geochemistry,zircon U-Pb chronology and other research methods.The result shows the formation age of basalt-rhyolite,material source,petrogenesis,formation and tectonic environment,providing reliable petrology and geochronology evidence for the final closure time limit of the northern Ancient Asian Ocean.This study mainly obtains the following understanding.?1?The LA-ICP-MS zircon U-Pb age of the rhyolite in the Jinta Formation is 265.3±4.3Ma,belonging to the middle Permian.We revise the age of Jinta Formation from early Permian to middle Permian.?2?Basalt-rhyolite assemblages are produced by cross-strata in the section,in which the basalt SiO₂ content ranges between 50.56-54.22wt%,and the rhyolite is 69.31-70.45wt%.There are obvious SiO₂ component discontinuity between basalt and rhyolite,and there are obvious differences in K₂O,Na₂O,MgO,TiO₂ content and so on,which is bimodal volcanic rocks of calc-alkali series.?3?In the basalt-rhyolite bimodal volcanic rocks,the primitive-mantle normalized trace element spidergram for the the basalts shows that as a whole basalts are enriched with large ion lithophilic elements Cs,Sr,Rb and Ba,and depleted with high-field strength elements Nb,Th and Ta.The chondrite-normalized rare earth element?REE?diagram shows that REE is nearly flat distribution,?La/Yb?_N=1.5,no Eu anomalies to slight positive anomalies,and?Eu=1.05-1.20.Rhyolite weakly enriched large ion lithophilic elements?LILE?and LREE,with obvious depletion of high field strength elements Th,Nb,Ta,Zr,Hf,Ti,P,Sr and HREE,?La/Yb?_N=6.0,and medium negative Eu anomalies,?Eu=0.61-0.79.Combined with regional geological background,the comprehensive study shows that the basalt originates from the partial melting of the depleted mantle replaced by the previously subduction fluid,and the rhyolite originates from the partial melting of the lower crust material by the basalt magma underplating.Basalt and rhyolite are non-homologous.?4?Comprehensive the mineralogy,petrology and geochemistry characteristic of the basalt-rhyolite biomodal volcanic rocks,combining with previous research results,we can find out that the basalt-rhyolite bimodal volcanic rocks of Jinta Formation in north to Alxa Block formed in the post-orogenic extension environment,and the closure time of the southern Ancient Asian Ocean should be before middle Permian,and the study area has entered the post-orogenic extension environment in middle Permian.